Ofdm transmittter and ofdm receiver

ABSTRACT

In an environment with large transmission delays, use of an OFDM transmitter which includes: a pilot/data allocator for allocating pilot/data symbols on OFDM symbols and an OFDM receiver which includes: an antenna for receiving the OFDM signals sent out from the antenna of this OFDM transmitter; a ratio unit for frequency transforming the OFDM signals received as RF signals to baseband signals; a frequency offset estimate for estimating an offset value; and a frequency offset corrector for performing frequency compensation by the amount of the frequency offset, improves data transmission efficiency while reducing interference of data between sub-carriers to prevent degradation of reception characteristics by performing appropriate frequency offset correction.

TECHNICAL FIELD

The present invention relates to an OFDM transmitter and OFDM receiverin a communication system, based on OFDM technique, and usingcommunication technologies such as OFDM technique, MIMO and the like ina combination, in which pilot symbols provided inside OFDM symbols areused to estimate the carrier frequency offset between the transmitterand the receiver.

BACKGROUND ART

In recent years, studies on mobile communication schemes based on OFDMtechnique, or other communication schemes, for example a mobilecommunication scheme based on the combination of a communicationtechnique such as MIMO, CDMA or the like and this OFDM technique, havebeen actively conducted. The adoption of OFDM technique has also beendecided as the downlink communication scheme in LTE (Long TermEvolution) that investigates the next-generation specifications in 3GPP(The 3rd Generation Partnership Project) for setting the standard ofmobile phones.

In particular, in OFDM signal mobile communication systems handlingmultimedia information etc., support for various levels of quality hasbeen requested. For example, in multimedia digital communications usingmobile information terminals, highly reliable signal transmission isrequired while securing the convenience of mobile communication thatenables access to the communication networks and the like from anarbitrary point.

Here, in digital communications, not limited to mobile communications,it is necessary to establish frequency synchronization between thetransmitter and the receiver in order to restore the originalinformation transmitted from the transmitter. In particular, in mobilecommunications, the synchronization process is indispensable because thereceiving condition varies. However, synchronization establishment needsa certain period of time. In a state where synchronization isunestablished, it is impossible to restore the original information,hence high-speed frequency synchronization is needed also for recoveryfrom the state of being out of synchronization.

In OFDM signal communication systems handling multimedia informationetc., packet communication is used since the information to betransmitted can be generated in burst modes. In packet communications,signals are not transmitted continuously or with regular intervals, butare transmitted at a burst when information to be transmitted takesplace. Accordingly, it is necessary to establish synchronization everyburst, and establish synchronization in a short time.

Further, in mobile data terminals handling multimedia information, sinceit is difficult to employ a high-precision oscillator in view ofminiaturization, it is necessary to apply a high-performancecarrier-frequency synchronizing method.

By the way, the OFDM transmission scheme is a scheme in whichinformation to be transmitted is spitted into multiple digital signalsand the multiple signals are used to modulate sub-carriers which areorthogonal to each other. This parallel transmission using thesesubcarriers enables the reduction of the signal transmission rate, andprovision of guard intervals, which feature OFDM, enables the reductionof the influence of delayed waves compared to a single carriermodulation scheme.

At the same time, in the transmission scheme based on OFDM technique,since the sub-carrier spacing is set small, if there exists a carrierfrequency deviation (offset) between the transmitter and receiver, theorthogonality between sub-carriers collapses and interference takesplace. Therefore, this scheme is known to undergo sharp degradation ofreception characteristics compared to other transmission schemes(reference literature: Minoru Okada, “Basis of OFDM”, Microwave Workshopand Exhibition (MWE2003), Tutorial Lecture 02, DigitalModulation/Demodulation Technology (2003-11)).

Accordingly, it is very important to establish carrier frequencysynchronization in the transmission scheme based on OFDM technique.

In order to solve the above problems, for example, the following patentdocument 1 discloses a technology in which, from OFDM symbols formed ofOFDM data in frame units, into which pilot symbols are inserted atregular intervals, and into which the aforementioned guard intervals areinserted for every pilot symbol and data symbol, a guard interval fordata symbols is detected so as to calculate the approximate frequencyoffset from sampled data in this interval and make compensation for it,then the fine frequency offset is calculated from the pilot symbols tomake compensation.

The following patent document 2 also discloses a technology forcalculating the frequency offset using data symbols in guard intervals.

Patent document 1:

Japanese Patent Application Disclosure 2003-503944

Patent document 2:

Japanese Patent Application Laid-open H09-102774

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, the pilot symbol area for calculating the fine frequency offsetdescribed in the above patent document 1 is all filled with pilotsymbols of a known sequence, and the pilot symbols are used to calculatethe frequency offset.

Accordingly, the technology in the aforementioned patent document 1 isnot applied to a format in which data symbols are inserted into thispilot symbol area, so that there is the problem that wireless resourcescannot be used efficiently, hence data transmission efficiently cannotbe improved.

Further, under an environment in which there are large transmissiondelays, the delayed waves penetrate into the guard intervals, hencethere occurs the case where the periodicity of the guard intervalsalmost disappears. Accordingly, there occurs such a problem thatestimation error in frequency offset compensation becomes large.

The technology disclosed in the aforementioned patent document 2 alsouses the data of the guard intervals, hence has the same problems asthose of the aforementioned patent document 1.

The present invention has been proposed in view of the abovecircumstances, it is therefore an object of the present invention toprovide an OFDM transmitter and OFDM receiver which efficiently usewireless resources to improve data transmission efficiency and at thesame time performs suitable frequency offset correction without usingdata symbols in guard intervals.

Means for Solving the Problems

In order to solve the above problems, the OFDM transmitter and OFDMreceiver according to the present invention have the characteristics asfollows.

An OFDM transmitter according to the present invention is an OFDMtransmitter for use in a communication scheme based on OFDM technologyor in a communication system using OFDM technology and anothercommunication technology, comprising: a pilot/data allocator forallocating pilot symbols of a predetermined known signal sequence anddata symbols, at predetermined positions in OFDM symbols; an IFFTprocessor for performing IFFT operation for the OFDM symbols output fromthe pilot/data allocator to generate OFDM signals in time domain; and, aradio unit for transmitting the OFDM signals via transmission carriersignals as RF signals, and wherein the pilot/data allocator allocates aplurality of sets including a plurality of the pilot symbols,equi-distantly in at least two ore more of the OFDM symbols, allots thepilot symbols in the set to adjoining sub-carriers and arrange the pilotsymbols closely, and allocates pilot symbols in the other OFDM symbolwhile keeping relative positional relationship with the pilot symbols inthe set.

The OFDM transmitter according to the present invention is alsocharacterized in that the pilot/data allocator distributes pilot symbolsin such an arrangement that the pilot symbols arranged in the OFDMsymbols are located line-symmetrically, taking the middle line of thefrequency axis of the OFDM symbols as the axis of symmetry.

The OFDM transmitter according to the present invention is alsocharacterized in that in the plural sets, the individual pilot symbolsin a first set respectively have pilot symbol values that are multipliedby a first common coefficient while the individual pilot symbols in asecond set respectively have pilot symbol values that are multiplied bya second common coefficient, and the pilot symbol series in the firstset and the pilot series in the second set are the same except thedifference between the common coefficients.

The OFDM transmitter according to the present invention is alsocharacterized in that the pilot/data allocator includes a data bufferunit for buffering transmission data, a pilot signal generator forgenerating pilot signals and a data switching control means forperforming switching control between the pilot signal and thetransmission data, and the data switching control means changes theallocation pattern of the pilot symbols, by modifying the timing ofswitching control between the pilot signal and the transmission data.

The OFDM transmitter according to the present invention is alsocharacterized in that the another communication technology is MIMO, andthe pilot/data allocator allocates multiple kinds of pilot symbolscorresponding to the number of transmitting antennas in the set.

An OFDM receiver according to the present invention is an OFDM receiverfor receiving the RF signals generated by the pilot/data allocator inthe OFDM transmitter defined in any one of Claims 1 to 5, comprising: aradio unit for converting the RF signals into the baseband to generatetime-domain OFDM signals; a frequency offset estimator for estimatingthe offset of a modulated carrier frequency between the transmitter andreceiver; and, a frequency offset corrector for performing frequencyoffset correction based on the frequency offset calculated from thefrequency offset estimator, and is characterized in that the frequencyoffset estimator includes: a FFT processor for generatingfrequency-domain OFDM symbols from the OFDM signals; a pilot processorwhich performs, of the generated OFDM symbols, complex correlatingoperations between a pilot symbol located at a particular sub-carrierfrequency in the m-th OFDM symbol in an OFDM frame and the pilot symbolslocated at sub-carrier frequencies a predetermined distance apart in twodirections, toward higher and lower positions, from the particularsub-carrier frequency in the n-th OFDM symbol, to output a complexcorrelation value; and a frequency offset calculator for calculating thefrequency offset based on the phase rotation quantity of the complexcorrelation value.

The OFDM receiver according to the present invention is alsocharacterized in that the pilot processor calculates the total averagequantity of phase rotation between the particular pilot symbols that arelocated adjacent to each other inside one of the OFDM symbols, andperforms the complex correlating operations by performing phasecorrection to the pilot symbols located at the particular sub-carrierfrequencies in the m-th OFDM symbol based on the total average quantityof phase rotation.

The OFDM receiver according to the present invention is alsocharacterized in that the pilot processor calculates the first averagequantity of phase rotation between the particular pilot symbol in them-th OFDM symbol and a pilot symbol located adjacent to the pilot symbolon the higher sub-carrier frequency side thereof, and the second averagequantity of phase rotation between the particular pilot symbol in them-th OFDM symbol and a pilot symbol located adjacent to the pilot symbolon the lower sub-carrier frequency side thereof, and performs thecomplex correlating operations by performing phase correction to thepilot symbol located at the particular sub-carrier frequency in the m-thOFDM symbol based on the first average quantity of phase rotation andthe second average quantity of phase rotation.

EFFECT OF THE INVENTION

Since the OFDM transmitter and OFDM receiver according to the presentinvention are configured as above, it is possible to provide the effectas follows.

According to the OFDM transmitter and OFDM receiver used in thecommunication system of the present invention, allocation of pilotsymbols inside OFDM symbols in such a manner as to reduce estimationerror in calculating the frequency offset by complex correlatingoperations, makes it possible to improve the accuracy of frequencyoffset calculation to reduce the interference of data betweensub-carriers, prevent the degradation of reception characteristics andcontribute to the improvement of channel estimation error using pilotsymbols.

According to the OFDM transmitter and OFDM receiver used in thecommunication system of the present invention, arrangement of both pilotsymbols and data symbols inside OFDM symbols, makes it possible to useradio resources effectively and enhance transmission efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system block diagram of an OFDM transmitter and OFDMreceiver according to the present invention.

FIG. 2 is a system block diagram of another OFDM transmitter and OFDMreceiver according to the present invention.

FIG. 3 is a system block diagram of another OFDM transmitter and OFDMreceiver according to the present invention.

FIG. 4( a) is a diagram showing a first pilot pattern and (b) is adiagram showing a second pilot pattern.

FIG. 5 is a block diagram showing the configuration of a pilot/dataallocator of an OFDM transmitter according to the present invention.

FIG. 6( a) is a diagram showing which pilot symbols (P1, P1′) are takento perform correlating operations in the first pilot pattern and (b) isa diagram showing which pilot symbols (P1, P1′) are taken to performcorrelating operations in the second pilot pattern.

FIG. 7 is a block diagram showing the configuration of a frequencyoffset estimator 202 of the present embodiment.

FIG. 8 is a block diagram showing the configuration of a pilot processor251 of the first example.

FIG. 9 is a block diagram showing the configuration of a pilot processor300 of the second example.

FIG. 10 is a block diagram showing the configuration of a pilotprocessor 350 of the third example.

FIG. 11 is a chart showing specifications for computer simulation.

FIG. 12 is a chart showing the simulation result based on a frequencyoffset estimating method 1.

FIG. 13 is a chart showing the simulation result based on a frequencyoffset estimating method 2.

FIG. 14 is a chart showing the simulation result based on a frequencyoffset estimating method 3.

DESCRIPTION OF REFERENCE NUMERALS

-   10 OFDM transmitter-   20, 21, 22 OFDM receiver-   30 Transmission path-   100 Pilot/data allocator-   101 Modulator-   102 IFFT processor-   103, 201 Radio unit-   104, 200 Antenna-   202, 212 Frequency offset estimator-   203 Frequency offset corrector-   204, 250 FFT processor-   205, 225 Channel estimator-   206 Demodulator-   240 Data buffer-   241 Pilot signal generator-   242 Data switching control means-   243 Changeover SW-   251, 300, 350 Pilot processor-   252, 264, 265, 266 Adder-   253 Phase transformer-   254 Frequency offset calculator-   26 Delay unit-   262 Complex conjugator-   263 Multiplier

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the embodiment of an OFDM transmitter and OFDM receiver accordingto the present invention will be described with reference to thedrawings.

FIGS. 1 to 14 show one exemplary embodiment of an OFDM transmitter andOFDM receiver according to the present invention. In the drawings, partsallotted with the same reference numerals are assumed to represent thesame components.

To begin with, the configuration and overall operation of acommunication system for the OFDM transmitter and OFDM receiver of thepresent invention will be briefly described.

FIG. 1 is a system block diagram showing an OFDM transmitter and OFDMreceiver according to the present invention.

Herein, a system example in which an OFDM communication scheme and a 4×4MIMO communication scheme are combined (4×4 MIMO-OFDM communicationsystem) will be described.

The 4×4 MIMO-OFDM communication system shown in FIG. 1 is a system fortransmitting and receiving a 4-branch OFDM signal via 4×4 MIMO channels(transmission paths 30) and is comprised of an OFDM transmitter 10 andan OFDM receiver 20.

Here, to be concise, the drawing only shows the constitutions related toOFDM transmitter 10 and OFDM receiver 20 of the present invention.

The arrangement of pilot symbols that characterizes the presentinvention, the detailed configurations and operations of a pilot/dataallocator 100 provided for OFDM transmitter 10 for allocating pilot anddata symbols on OFDM symbols, and a frequency offset estimator 202provided for OFDM receiver 20 for calculating a frequency offsetestimate based on the pilot symbols, will be described later.

OFDM transmitter 10 includes: the aforementioned pilot/data allocator100 for allocating pilot/data symbols on OFDM symbols; an unillustratedguard interval inserting block; a modulator 101 for modulatingsubcarriers with pilot/data signals from pilot/data allocator 100; anIFFT processor (Inverse Fast Fourier Transform processor) 102 fortransforming the modulated signal from frequency-domain signals intotime-domain signals; a radio unit 103 for converting OFDM signals thathave been transformed in time-domain representation into RF signals; andan antenna 104 for radiating the converted RF signals as radio waves totransmission path 30. Since a 4×4 MIMO-OFDM communication system ispresumed herein, OFDM transmitter 10 is provided with four sets of theabove identical series.

On the other hand, OFDM receiver 20 that receives OFDM signalscontaining pilot signals, sent out from OFDM transmitter 10 viatransmission path 30, includes: an antenna 200 for receiving OFDMsignals sent out from antennas 104 of OFDM transmitter 10; a radio unit201 for performing frequency conversion of OFDM signals received as RFsignals into baseband signals; a frequency offset estimator 202 fordetecting a modulated carrier frequency deviation (offset) between theOFDM transmitter and receiver to estimate the offset value; a frequencyoffset corrector 203 for making frequency compensation by the amount offrequency offset estimated by frequency offset estimator 202; a FFT(Fast Fourier Transform processor) 204 for transforming the time-domainsignal that has been subjected to frequency compensation throughfrequency offset corrector 203 into its frequency-domain signal; achannel estimator 205 for compensating for channel gain variations dueto change of communication environment of the channel; and a demodulator206 for demodulating the OFDM signals to output the transmitted data.

Since a 4×4 MIMO-OFDM communication system is constructed, similarly tothe OFDM transmitter, the receiver is also provided with four sets ofthe series from radio unit 201 to FFT processor 204.

Next, the overall operation of the transmitter/receiver system thusconstructed as above will be described briefly.

OFDM transmitter 10 forms separate OFDM signals corresponding to fourtransmission antennas while OFDM receiver 20 receives the OFDM signalsby four receiving antennas and converts the OFDM signal in the RF signalform into its baseband time-domain OFDM signal for every receivingantenna. Then, frequency offset estimator 202 calculates a frequencyoffset estimate for every OFDM frame to perform frequency offsetcorrection by frequency offset corrector 203. Here, the OFDM frame is asignal unit consisting of a plurality of OFDM symbols, and does notnecessarily coincide with the processing unit of transmission data.

In calculating the frequency offset estimate, as described above, inorder to prevent degradation of reception characteristics due tooccurrence of interference caused by loss of the orthogonality betweensub-carriers on the receiver side as a result of the sub-carrierfrequency deviation (offset) between the OFDM transmitter and receiverand in order to improve transmission efficiency, OFDM transmitter 10inserts data also into the pilot symbols of OFDM transmitter 10, andalso generates a pilot pattern within the pilot symbols that enable thereceiver side to perform frequency offset estimation with a goodprecision and sends out the pilot symbols based on this pilot pattern asthe OFDM signal to the receiver side. Frequency offset estimator 202 inOFDM receiver 20 detects and extracts the pilot symbols and implements acorrelating process between aftermentioned two pilot symbols tocalculate the frequency offset value.

Further, in OFDM receiver 20, the OFDM signal in frequency time-domainis transformed into a frequency-domain OFDM by FFT processor 204, thenestimation and compensation of channel gain are performed by channelestimator 205, and the demodulated data is obtained by demodulator 206.

Though it is usual that the operation of channel gain estimation bychannel estimator 205 is also performed every one frame, the processingunit for offset estimation and the processing unit for channel gain maybe different.

Further, frequency offset estimator 202 of the present embodiment isconfigured to directly perform frequency compensation for thetime-domain OFDM signal, using frequency offset corrector 203, but it isalso possible to provide a configuration in which an AFC (automaticfrequency control) operation for converging the frequency error by aloop process is functioned using the calculated frequency offsetestimate in a synthesizer unit 213, as shown in FIG. 2. However, in thecase of the AFC operation, a time lag occurs to reflect the correctionby the estimated frequency offset due to the loop process. Also, theconvergence time usually becomes greater compared to the case of thepresent embodiment.

Further, channel estimator 205 calculates the channel gain estimateusing pilot symbols in frequency-domain, but when frequency correctionis performed based on the frequency offset estimate obtained byfrequency offset estimator 22, interference of data between sub-carriersis reduced, so that channel estimate error can be improved.

As shown in FIG. 3, it is also possible to construct a channel estimator225 that improves channel estimation accuracy by use of the result etc.obtained midway from frequency offset estimator 202.

Next, a configurational example of a pilot pattern for improving datatransmission efficiency and for calculating the frequency offset withgood precision will be demonstrated, and the configuration and operationof a pilot pattern generator in the OFDM transmitter will be describedhereinbelow.

The data configuration of an OFDM frame is usually formed of datasymbols and pilot symbols transmitted as a signal of a known sequence.FIG. 4 is a diagram showing a configurational example of a pilot patternin the OFDM frame according to the present invention, (a) showing apilot pattern 1 and (b) showing a pilot pattern 2.

The pilot patterns shown in FIGS. 4( a) and (b) are constructed aimingat allocating pilot symbols that can improve data transmissionefficiency, enables calculation of the frequency offset with a goodprecision on the OFDM receiver side, and still does not need eitherdevice scale or complex configuration. Since the pilot pattern shown inFIG. 4( a) is arranged so that pilots are put together, this results inan allocation that enables calculation of the frequency offset with afurther improved precision.

FIG. 4 shows one frame OFDM data in a two-dimensional representation, inwhich 64 sub-carrier frequencies are taken in the vertical direction and7 OFDM symbols are taken in the time axis direction (a matrix of 7 OFDMsymbols×64 subcarrier frequencies) and pilot symbols are distributedkeeping relative positional relationships in the sub-carrier axisdirection, inside the first time-axis and fifth time-axis OFDM symbols.

Since in the present embodiment, a 4×4 MTMO-OFDM communication system ispresumed as mentioned already, pilot symbols corresponding to fourtransmitting antennas are represented by P1, P2, P3 and P4,respectively. These pilot symbols represented by P1 to P4 areorthogonalized by allotting them to different sub-carriers, so that thesignal for each antenna will not interfere with the others. Here, Drepresents a data symbol.

The frequency offset estimating method according to the presentinvention (which will be detailed later) is to calculate a frequencyoffset estimate by examining the correlation between two pilot symbols(e.g., the m-th OFDM symbol and the n-th OFDM symbol) within theaforementioned one frame to calculate the quantity of phase rotation ofthe correlation value, thereby calculating the frequency offsetestimate.

As understood from the drawing, while two pilot symbols are provided,the ordinary data is allotted to the sub-carriers other than those ofpilot symbols (P1 to P4) in the pilot symbol areas, so that it ispossible to realize the reduction of the overhead and enhance datatransmission efficiency.

The difference between pilot patterns 1 and 2 shown in FIGS. 4( a) and(b) is that P1 to P4 are arranged contiguously or P1 to P4 are scatteredequally apart from each other. Since data symbols depends on thetransmission data sequence and what symbols they will be cannot beexpected, it is impossible to suppress interference between adjacentsub-carriers within a fixed level if there is a frequency offset. On theother hand, since pilot symbols can be constructed as a known signalsequence, it is possible to suppress interference between sub-carrierswithin a substantially fixed level. Accordingly, for example, the levelsof sub-carrier interference between pilot symbols in time axis columns 1and 5 become substantially equal to each other, it is hence possible toperform frequency offset estimation by conducting a correlating processbetween the two pilot symbols.

Since, in conducting a correlating process between pilot symbols in thefirst and fifth symbol columns, every pilot symbol (P1 to P4) isarranged symmetrically (arranged line symmetrically taking the middlesub-carrier frequency slot of OFDM symbols as the center axis) in such amanner that the number of additions in the correlating process betweenthe first pilot symbol to be the reference and the pilot symbol locatedin the fifth pilot symbol column and on the sub-carrier above the pilotsymbol to be the reference in the first pilot symbol column, and thenumber of additions in the correlating process with the pilot symbollocated in the fifth pilot symbol column and on the sub-carrier beloware as a whole equal to each other, it is possible to reduce error inphase rotation quantity operations, hence improve frequency offsetestimation accuracy.

Further, one set (block) of pilot symbols shown in the figure, forexample, in the block (the first set) designated by symbol A and in theblock (the second set) designated by symbol B, common coefficients k_(A)(the first coefficient) and k_(B) (the second coefficient) are definedinside respective blocks so that the pilot symbols in respective blocksmay be set as (k_(A)·P1, k_(A)·P2, k_(A)·P3, k_(A)·P4) and (k_(B)·P1,k_(B)·P2, k_(B)·P3, k_(B)·P4). For example, when thinking about P1, ifthe above relationship holds the interference component given from P2 toP1 is P2/P1, so that P1 will be affected by the common interferencecomponent, not depending on the blocks. Accordingly, the interferencebetween pilot sub-carriers can be canceled out when correlatingoperations are carried out, hence frequency offset estimation accuracycan be improved.

Next, the configurational example and the operation of a pilot/dataallocator of OFDM transmitter 10 will be described hereinbelow.

FIG. 5 is one example of a block diagram of the OFDM transmitteraccording to the present invention, and is a block diagram showing apilot/data allocator in OFDM transmitter 10 in the system block diagramof FIG. 1.

Pilot/data allocator 100 includes; a data buffer 240 for bufferingtransmitted data from the outside; a pilot signal generator 241 forgenerating a known signal sequence to be the pilot symbols; a dataswitching control means 242 for switching its output between data andpilots appropriately to send out a signal to modulator 101; and achangeover SW 243 that is controlled by data switching control means242.

Data switching control means 242 has a parameter table in which thetimes at which pilot signals are inserted on the one-frame format andinformation as to which antenna is used for transmitting each pilot havebeen recorded beforehand. In pilot signal generator 241, the pattern ofpilots to be generated in accordance with the instructions from dataswitching means 242 has been stored. In this arrangement, data switchingcontrol means 242, based on the data stored in this parameter table,switches its output between data and pilots to supply a suitablyarranged signal sequence to modulator 101.

In this way, it is possible to generate a pertinent pilot pattern.Further, it is also possible to generate an arbitrary arrangement ofpilots, not limited to the pilot allocations shown in FIGS. 4( a) and(b) when the above parameter table is modified.

Next, taking a configurational example of a frequency offset estimatorwhich receives the pilot signals based on the pilot pattern, sent outfrom OFDM transmitter 10 to perform frequency offset estimation, itsspecific operation will be described hereinbelow.

FIG. 7 is a block diagram showing a configurational example of frequencyoffset unit 202.

Frequency offset unit 202 includes FFTs 250 receiving time-domain OFDMsignals that are affected by carrier frequency offsets between thetransmitter and the receiver, convert the signals from time domain tofrequency domain to detect OFDM symbols in a predetermined time andpilot symbols inserted in the predetermined slot of the OFDM symbol onthe sub-carrier frequency axis; pilot processors 251 performing complexcorrelating calculation between pilot symbols (P1 to P4 in the firstcolumn of OFDM symbols and pilot symbols (P1′ to P4′) in the fifthcolumn of OFDM symbols in FIG. 4; an adder 252 adding up the correlationvalues; a phase transformer 253 performing a phase angle operation ofthe synthesized correlation value output from adder 252 to calculate asynthesized estimated phase difference θ between the first and fifthpilot symbols; and a frequency offset calculator 254 calculating afrequency offset from the synthesized estimated phase difference θbetween the first and fifth output from phase transformer 253 to outputthe frequency offset to frequency offset corrector 203 shown in FIG. 1.

A configurational example, correlating process and offset calculatingoperation of pilot processor 251 in frequency offset unit 202constructed as above, will be described using FIGS. 6, 8 to 10.

<Configurational Example of First Pilot Processor 251 (Frequency OffsetEstimating Method 1)>

FIG. 6 is a diagram showing between which pilot symbols (P1, P1′)correlating calculation should be done, by taking up pilot symbol P1after the FFT processing in FFT processor 250 shown in FIG. 7, (a) is adiagram showing between which pilot symbols (P1, P1′) correlatingcalculation should be done in the first pilot pattern and (b) is adiagram showing between which pilot symbols (P1, P1′) correlatingcalculation should be done in the second pilot pattern.

On the vertical axis or the sub-carrier frequency axis in FIG. 6, whenthe pilot symbols at which P1s are distributed are represented by r₀,r₁, . . . r_(Np-1), from the lowest sub-carrier frequency, the pilotsymbols on the first symbol column are located at r₀, r₂, . . . r_(Np-2)while the pilot symbols on the fifth symbol column are located at r₁,r₃, . . . r_(Np-1). In FIG. 6, the value of N_(P) is 16.

Here, there occurs a phase rotation from the phase of the sub-carrier ofpilot symbol P disposed in the first symbol to that of pilot symbol P′disposed in the fifth symbol in accordance with the carrier frequencyoffset Δf between the transmitter and receiver. This quantity of phaserotation can be determined by performing a complex correlating operationbetween pilot symbol P1 and pilot symbol P1′ to calculate the phaseangle of the complex correlation value. In order to calculate thequantity of phase rotation, the operation shown by the following formulais performed.

$\begin{matrix}{\theta = {\arg\left\lbrack {\sum\limits_{N}\; {\sum\limits_{M}\; \left( {{\sum\limits_{i = 0}^{{N\; {p/2}} - 1}{r_{{2\; i} + 1} \cdot r_{2\; i}^{*}}} + {\sum\limits_{i = 1}^{{N\; {p/2}} - 1}\; {r_{{2\; i} - 1} \cdot r_{2\; i}^{*}}}} \right)}} \right\rbrack}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The first term in the above formula is the complex correlation valuebetween the pilot symbol P1 in the first symbol column and the pilotsymbol P1′ (the pilot symbol one above the pilot symbol to provide thereference) located on the sub-carrier (designated by arrows a1 to a8 inFIGS. 6( a) and (b)) four levels above in the fifth symbol column. Thesecond term in the above formula is the complex correlation valuebetween the pilot symbol P1 in the first symbol column and the pilotsymbol P1′ (the pilot symbol one below the pilot symbol to provide thereference) located on the sub-carrier (designated by arrows b1 to b7 inFIGS. 6( a) and (b)) four levels below in the fifth symbol column.Σ_(n)Σ_(M) indicates the summation over all the transmitting antennasand receiving antennas. Here, in the above formula, r* indicates thecomplex conjugate of r and arg(x) indicates the phase angle of a complexnumber x.

In this way, it is possible to calculate the quantity of phase rotation(phase difference) between pilot symbols P1 and P1.

FIG. 8 is a block diagram showing a configuration of pilot processor 251of the first example.

When the pilot symbol P1 encircled in FIG. 8 is observed, thecalculation of the first term in the above formula 1 corresponds to thecomplex correlating operation between this pilot symbol P1 and the pilotsymbol P1′ located one thereabove and the calculation of the second termcorresponds to the complex correlating operation between this pilotsymbol P1 and the pilot symbol P1′ located one therebelow.

Further, in the above formula 1, the adding operations of thecorrelating calculation of the first and second terms are performedseparately from each other, but in pilot processor 251 of FIG. 8, theadding operations are performed simultaneously by a single adder 264.

Here, since pilot symbol 21 is obtained temporally ahead of pilot symbolP1′, the symbol is passed through a delay unit so as to be synchronizedwith P1′. Further, since for P1 in formula 1, its complex conjugate isproduced to be multiplied with P1′, a complex conjugator 262 is provideddownstream of delay unit 261.

Frequency offset estimator 202 of the present embodiment shown in FIG.7, includes sixteen pieces of pilot processors 251 to support 4transmitting antennas×4 receiving antennas, and adds up the correlationvalues obtained from these pilot processors 251 by means of a singleadder 252 and calculates the synthesized phase difference θ by phasetransformer 253 that performs an arg( ) operation on the resultantsynthesized correlation value in the same manner as in the above formula1.

The frequency offset value Δf indicated in the following formula iscalculated from the thus obtained phase difference.

$\begin{matrix}{{\Delta \; f} = \frac{\theta}{2\; {\pi \cdot {Ds} \cdot {Ts}}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Here, Ts is the OFDM symbol length, Ds is the interval between pilotsymbols (in this case Ds=5).

The thus calculated frequency offset value Δf is supplied to frequencyoffset corrector 203 shown in FIG. 1. This frequency offset corrector203 shifts the frequency by the amount of the frequency offset.

It is also possible to rewrite the above formula 1 into the form asfollows.

$\begin{matrix}{\theta = {\arg\left\lbrack {\sum\limits_{N}\; {\sum\limits_{M}\; \left( {{\sum\limits_{i = 1}^{N\; {p/2}}\; {\left( {r_{{2\; i} - 1} + r_{{2\; i} + 1}} \right) \cdot r_{2\; i}^{*}}} + {r_{1} \cdot r_{0}^{*}}} \right)}} \right\rbrack}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

This formula can be translated as that, for one pilot symbol P arrangedin the first symbol column, the average of pilot symbols P′ located atthe sub-carrier frequency four levels above and the sub-carrierfrequency four levels below in the fifth symbol column is determined tocalculate a channel estimate for the same sub-carrier frequency as thatof the pilot symbol P disposed in the first pilot symbol while thechannel estimate in the first symbol column is determined, whereby thephase difference is determined based on the channel estimate.

<Configurational Example of Second Pilot Processor 300 (Frequency OffsetEstimating Method 2)>

FIG. 9 is a block diagram showing a configuration of a pilot processor300 of the second example.

The configuration of this pilot processor 300 can be easily understoodfrom the following estimating method, so that the description is omittedherein.

To begin with, phase average φ of those between adjacent pilots(designated by arrows c1 to c7 and d1 to d7 in FIG. 6) in the same OFDMsymbol can be calculated by the formula below. That is, phase average φis calculated as:

$\begin{matrix}{\varphi = {\arg \left( {\sum\limits_{i = 0}^{{N\; p} - 3}\; {r_{i + 2} \cdot r_{i}^{*}}} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Then, the complex correlating operation between pilot symbol P1 and P1′at the subcarrier four levels thereabove by first pilot processor 251 ismodified by the phase-correcting component (−φ/2; the amount of phasedelay) and the complex correlating operation between pilot symbol P1 andP1′ at the subcarrier four levels therebelow is modified by thephase-correcting component (+φ/2; the amount of phase delay), so as tocalculate the quantity of phase rotation.

The formula for calculating the quantity of phase rotation is shownbelow.

$\begin{matrix}{\theta = {\arg\left\lbrack {\sum\limits_{N}\; {\sum\limits_{M}\; \begin{pmatrix}{{{\exp \left( {{- j}\; {\varphi/2}} \right)} \cdot {\sum\limits_{i = 0}^{{N\; {p/2}} - 1}{r_{{2\; i} + 1} \cdot r_{2\; i}^{*}}}} +} \\{{\exp \left( {j\; {\varphi/2}} \right)} \cdot {\sum\limits_{i = 1}^{{N\; {p/2}} - 1}\; {r_{{2\; i} - 1} \cdot r_{2\; i}^{*}}}}\end{pmatrix}}} \right\rbrack}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

According to this estimating method, execution of phase correction incomplex correlating operations makes it possible to further reduceestimation error compared to the estimate obtained by the aforementionedfrequency offset estimating method 1.

As shown in FIG. 9, phase average component θ11 is calculated by complexconjugators 262, multipliers 263, an adder 265 and a phase transformer253 in pilot processor 300. This phase average component ƒ11 correspondsto the aforementioned phase average φ. In the arrangement for phasecorrection to complex correlating operations, in order to performcorrection to the complex correlating operation between the pilot symbolP1 (i-th pilot symbol) and P1′((i+1)-th pilot symbol) at the subcarrierfour levels thereabove, multiplier 263 multiplies pilot symbol P1 by thephase correcting component (e^(−jθ11/2); corresponding to the above−φ/2)while in order to perform correction to the complex correlatingoperation between the pilot symbol P1 and P1′ at the subcarrier fourlevels therebelow, multiplier 263 multiplies pilot symbol P1 by thephase correcting component (e^(+jθ11/2); corresponding to the above+φ/2).

<Configurational Example of Third Pilot Processor 350 (Frequency OffsetEstimating Method 3)>

FIG. 10 is a block diagram showing a configuration of a pilot processor350 of the third example.

Here, the configuration of this pilot processor 350 can also be easilyunderstood from the following estimating method, similarly to theconfiguration of the above second pilot processor 350, so that thedescription is omitted herein.

In pilot processor 300 of the above second example, to calculate theamount of phase correction, all the first pilot symbols (P1) and thesecond pilot symbols (P1′) are used (the same is also performed forother P2 to P4) to calculate phase correcting component θ11, wherebycorrection to pilot symbol P1 with (±θ11/2) is carried out. In contrast,pilot processor 350 of the present embodiment is configured so as tocorrect the phase of pilot symbol P1 by calculating phase correctingcomponents (θ11 b, θ11 c) for every sub-carrier frequency component, forpilot symbol P1 in the first symbol column.

In correcting pilot symbol P1, there is a fear that estimation errorbecomes rather greater due to various phase conditions when phasecorrection is performed using phase correcting component θ11. Therefore,the present example is configured to perform phase correction based onthe phase correcting components that are calculated using only the pilotsymbol P1 as the reference for correlating operations.

As the configurations of the frequency offset estimators and differentfrequency estimating methods have been described heretofore, it is foundthat from the result of evaluation on frequency offset estimation errorby performing computer simulations of the different estimating methods,any of the methods could produce estimation within the predeterminedrange. FIG. 11 shows the specifications used for evaluation by computersimulation. FIGS. 12 to 14 show the simulation results by differentfrequency offset estimating methods.

Though the present embodiment has been described using OFDM-MIMOcommunication systems, the present embodiment can be applied to the casewhere, for example, transmission diversity is performed in a system withfour transmitting antennas and one receiving antenna. Alternatively, itis also possible to apply the embodiment to an OFDM communication systemwith one transmitting antenna and one receiving antenna, by transmittingP1 and P2 from the transmitting antenna.

The OFDM transmitter and OFDM receiver used in the ODFM signal mobilecommunication system according to the present invention are not limitedto the above described embodiment modes, but it goes without saying thatvarious modifications can be added without departing from the scope ofthe present invention.

Since the OFDM transmitter and OFDM receiver used in the ODFM signalmobile communication system according to the present invention canimprove the accuracy of frequency offset calculation to reduceinterference of data between sub-carriers, prevent degradation ofreception characteristics and contribute to improvement of channelestimation error using pilot symbols, and at the same time enableseffective use of radio resources to improve data transmissionefficiently, it is possible to widely apply them to the mobilecommunication systems and the like, for which highly reliable signaltransmission is required.

1. An OFDM transmitter for use in a communication scheme based on OFDMtechnology or in a communication system using OFDM technology andanother communication technology, comprising: a pilot/data allocator forallocating pilot symbols of a predetermined known signal sequence anddata symbols, at predetermined positions in OFDM symbols; an IFFTprocessor for performing IFFT operation for the OFDM symbols output fromthe pilot/data allocator to generate OFDM signals in time domain; and, aradio unit for transmitting the OFDM signals via transmission carriersignals as RF signals, wherein the pilot/data allocator allocates aplurality of sets including a plurality of the pilot symbols,equi-distantly in at least two or more of the OFDM symbols, allots thepilot symbols in the set to adjoining sub-carriers and arrange the pilotsymbols closely, and allocates pilot symbols in the other OFDM symbolwhile keeping relative positional relationship with the pilot symbols inthe set.
 2. The OFDM transmitter according to claim 1, wherein thepilot/data allocator distributes pilot symbols in such an arrangementthat the pilot symbols arranged in the OFDM symbols are locatedline-symmetrically, taking the middle line of the frequency axis of theOFDM symbols as the axis of symmetry.
 3. The OFDM transmitter accordingto claim 1, wherein in the plural sets, the individual pilot symbols ina first set respectively have pilot symbol values that are multiplied bya first common coefficient while the individual pilot symbols in asecond set respectively have pilot symbol values that are multiplied bya second common coefficient, and the pilot symbol series in the firstset and the pilot series in the second set are the same except thedifference between the common coefficients.
 4. The OFDM transmitteraccording to claim 1, wherein the pilot/data allocator includes a databuffer unit for buffering transmission data, a pilot signal generatorfor generating pilot signals and a data switching control means forperforming switching control between the pilot signal and thetransmission data, and wherein the data switching control means changesthe allocation pattern of the pilot symbols, by modifying the timing ofswitching control between the pilot signal and the transmission data. 5.The OFDM transmitter according to claim 1, wherein the anothercommunication technology is MIMO, and the pilot/data allocator allocatesmultiple kinds of pilot symbols corresponding to the number oftransmitting antennas in the set.
 6. An OFDM receiver for receiving theRF signals generated by the pilot/data allocator of the OFDM transmitterdefined in claim 1, comprising: a radio unit for converting the RFsignals into the baseband to generate time-domain OFDM signals; afrequency offset estimator for estimating the offset of a modulatedcarrier frequency between the transmitter and the receiver; and, afrequency offset corrector for performing frequency offset correctionbased on the frequency offset calculated from the frequency offsetestimator, wherein the frequency offset estimator includes: a FFTprocessor for generating frequency-domain OFDM symbols from the OFDMsignals; a pilot processor which performs, of the generated OFDMsymbols, complex correlating operations between a pilot symbol locatedat a particular sub-carrier frequency in the m-th OFDM symbol of an OFDMframe and the pilot symbols located at sub-carrier frequencies apredetermined distance apart in two directions, at higher and lowerpositions, from the particular sub-carrier frequency in the n-th OFDMsymbol, to output a complex correlation value; and a frequency offsetcalculator for calculating the frequency offset based on the phaserotation quantity of the complex correlation value.
 7. The OFDM receiveraccording to claim 6, wherein the pilot processor calculates the totalaverage quantity of phase rotation between the particular pilot symbolsthat are located adjacent to each other inside one of the OFDM symbols,and performs the complex correlating operations by performing phasecorrection to the pilot symbols located at the particular sub-carrierfrequencies in the m-th OFDM symbol based on the total average quantityof phase rotation.
 8. The OFDM receiver according to claim 6, whereinthe pilot processor calculates the first average quantity of phaserotation between the particular pilot symbol in the m-th OFDM symbol anda pilot symbol located adjacent to the pilot symbol on the highersub-carrier frequency side thereof, and the second average quantity ofphase rotation between the particular pilot symbol in the m-th OFDMsymbol and a pilot symbol located adjacent to the pilot symbol on thelower sub-carrier frequency side thereof, and performs the complexcorrelating operations by performing phase correction to the pilotsymbol located at the particular sub-carrier frequency in the m-th OFDMsymbol based on the first average quantity of phase rotation and thesecond average quantity of phase rotation.